1. Field of the Invention
The present invention relates to an image-forming device, and particularly to a tandem image-forming device.
2. Description of Related Art
Conventional tandem image-forming devices include photosensitive drums, drum gears being provided on the ends of the photosensitive drums, and drum drive gears and the like disposed on the image-forming device for driving the drum gears. However, these drum gears and drum drive gears are difficult to manufacture without errors in shape or dimension.
Such errors in the shape or dimension of the drum gear and drum drive gear cause the photosensitive drum on which the drum gear is mounted to rotate irregularly. When the image-forming device is a tandem-type device having a plurality of photosensitive drums, irregular rotations of the photosensitive drums may cause color registration problems in the image.
Since these photosensitive drums are generally treated as consumables that are discarded after use, the drum gear disposed on the end of the photosensitive drum is normally formed of a resin material in order to minimize the manufacturing cost of the photosensitive drum. However, due to the physical properties of the resin, a resin gear is even more prone to errors in shape and dimension.
To resolve this problem, an image-forming device proposed in Japanese unexamined patent application publication No. 2003-43780 tolerates the inevitable irregular rotations in the photosensitive drums and forms images by constantly synchronizing the image transfer position in the image transfer unit of each drum relative to one another. The proposed construction of this image-forming device is an effort to eliminate problems in color registration among the superimposed images.
In this construction, the engaged positions of drum gears to drum drive gears is adjusted in the factory prior to shipping the image-forming device to ensure that the rotational position (phase) is the same for each of the photosensitive drums. However, when a photosensitive drum has reached the end of its life and must be replaced, the drum gear mounted on each photosensitive drum must be replaced together with the photosensitive drum. Consequently, the new drum gear will likely engage with the drum drive gear at a different position.